Until recently, sightings of this rarely-seen Scorpion-tailed Spider (Arachnura melanura) could have been counted on one hand, then approximately fifty of them were found on one tree in our tropical fruit orchard. A star-apple fruit tree (Chrysophyllum cainito), with an abundant growth of epiphytes, was loaded, but given the amazing mimicry of a dead leaf, it is possible that in the past they were merely discarded without proper scrutiny.
Arachura melanura builds its relatively fine-meshed orb web on the lower branches of bushes where there are small gaps in the outer layer of foliage. The construction lies across the flight path of insects, flying into or out of the inner non-leafy part of the trunk area. The conventional meshed orb web is virtually invisible, with each sector either side of the 12 o’clock radius free of spiral elements.
A. melanura sits at the closed hub of the web, beneath a string of plant debris and discarded exoskeletons occupying the vacant upper sectors either side of the upper radius. Mature females string their egg cocoons within this alignment of clutter. The primary defence of the female spider applies diurnally, when she appears as a dead leaf. The elaborately enhanced tail magnificently mimics a leaf stem or petiole, particularly as it extends into the broader character of the abdomen and continues into the strategically held legs. Forward protraction of the front four legs ahead of the body, taper to a centre-point that conforms to the edge and ultimately the drip-point of a typically suspended leaf. The rear legs are drawn inconspicuously against the sides of the abdomen, bent slightly so that their tarsi grip the web-hub.
Two colour morphs seem to exist, with bright yellow and more commonly, white/brown. The yellow form could be regarded as a ‘less-dead’ leaf, with perhaps greater strength as a visual cue to increase attractiveness to nocturnal insects for a greater capture-rate at night. The brown form is correspondingly less likely to be seen by bird predators during the day, but less successful in food capture at night. Diurnally, spiders of both colour form align with the egg-sacs and plant detritus and disappear as part of the clutter.
As the common name implies, the look of the tail and some of its use when threatened, suggests an additional defensive strategy. If threatened, the spider resorts to high intensity web-shaking as it oscillates back and forth about a point somewhere near its spinnerets. The tail of the spider moves up and down conspicuously in a manner not unlike a scorpion. This appears to be the nocturnal defence to threats from other spiders, whilst diurnal threats from birds rely on dead-still leaf mimicry.
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Cooper Creek Wilderness
2333 Cape Tribulation Rd.,
Diwan QLD 4873